Cetrifugal blower system and fuel cell incorporating same

ABSTRACT

A centrifugal blower system includes a series of blower units, each blower unit in the series comprising a casing having an axial inlet and a radial outlet, an impeller disposed within the casing for drawing a gaseous medium at a first pressure into the inlet and expelling gaseous medium at a second higher pressure through the outlet and a motor for driving the impeller; and, a duct connecting the outlet of at least one blower unit in the series with the inlet of at least one other blower unit in the series.

REFERENCE TO PRIOR APPLICATIONS

This application is a divisional application of application Ser. No.13/168,280, filed Jun. 24, 2011, the entire contents of which areincorporated herein by reference.

BACKGROUND OF THE INVENTION

This invention relates to centrifugal blowers and to fuel cellsincorporating same.

Centrifugal blowers, or centrifugal fans, are a well known type ofdevice for providing a flow or movement of a gaseous medium. A commontype of centrifugal blower includes a housing having an axially directedgas inlet and a radially directed gas outlet, an impeller disposedwithin the housing for drawing gas at a first pressure into the inletand expelling gas at a second higher pressure through the outlet and amotor for driving, i.e., spinning, the impeller. Variations of thisgeneral type of centrifugal blower are disclosed in, e.g., U.S. Pat.Nos. 4,917,572; 5,839,879; 6,877,954; 7,061,758; 7,351,031; 7,887,290;7,891,942, and, U.S. 2006/0051203, the entire contents of which areincorporated by reference herein.

Centrifugal blowers in single unit and multiple independent unitconfigurations have been disclosed as components of cooling systems forcomputers, servers and other heat-generating electrical and electronicdevices and equipment. See, e.g., U.S. Pat. Nos. 6,525,935; 7,184,265;7,744,341; 7,802,617; 7,864,525; 7,885,068; 7,948,750; 7,902,617; and,7,885,068, the entire contents of which are incorporated by referenceherein.

Centrifugal blowers of the general type referred to above have beendisclosed as components of fuel cells, of both the polyelectrolytemembrane (PEM) and solid oxide fuel cell (SOFC) types, where theyfunction in one or more capacities, e.g., providing a flow of anoxidizer-containing gas such as air to the cathode elements of the fuelcell assembly and/or a flow of gaseous or vaporized fuel to its anodeelements, recycling unspent fuel to the anode elements of the fuel cellassembly, providing a stream of cool air for cooling the fuel cellassembly or providing a stream of hot gas for vaporizing a liquid fuelprior to the external or internal reforming of the fuel to providehydrogen for the operation of the fuel cell assembly. Fuel cell-blowerassemblies featuring one or more centrifugal blowers are described in,e.g., U.S. Pat. Nos. 6,497,971; 6,830,842; 7,314,679 and 7,943,260, theentire contents of which are incorporated by reference herein.

SUMMARY OF THE INVENTION

In accordance with the present invention, there is provided acentrifugal blower system comprising:

a) a series of blower units, each blower unit in the series comprising acasing having an axial inlet and a radial outlet, an impeller disposedwithin the casing for drawing a gaseous medium at a first pressure intothe inlet and expelling gaseous medium at a second higher pressurethrough the outlet and a motor for driving the impeller; and

b) a duct connecting the outlet of at least one blower unit in theseries with the inlet of at least one other blower unit in the series.

Further in accordance with the present invention there is provided afuel cell comprising:

a) a fuel cell assembly comprising a plurality of individual fuel cellseach fuel cell having an electrolyte medium, a cathode and an anode;and,

b) at least one centrifugal blower system, described supra, forproviding a flow of gaseous medium to the fuel cell assembly.

The multiple centrifugal blower system herein offers several advantagesover a single centrifugal blower, particularly when incorporated in afuel cell for managing the flow of gaseous media therein.

Single centrifugal blowers require suitable control of the full range ofmotor rpm in order to meet fluctuating gas flow demands. Depending onthe pressure and flow requirements for a particular blower application,optimum performance of the blower may be achieved by employing animpeller of relatively small size driven at relatively high rpm, e.g.,20,000 rpm and above, or an impeller of relatively large size driven atrelatively low rpm, e.g., below 20,000 and more commonly, below 10,000.The first arrangement, i.e., the use of a relatively small impellerdriven at relatively high rpm, requires a more powerful and specializedmotor which of necessity will draw a correspondingly greater amount ofpower for its operation. The second arrangement, i.e., use of arelatively large impeller driven at relatively low rpm, makes controland fine tuning of the blower output more difficult due to the greaterinertia of a large impeller.

In order to prevent overshoot of the target pressure and gas flow, ablower employing a relatively high inertia impeller must be overdampedwhen tuning the blower for its expected range of gas pressure and flowcapability. The effect of this overdamping to compensate for therelatively high inertia of the impeller is to cause the blower to beslow in responding to changing, and often rapidly changing, gas flowrequirements. This characteristically slow response of a singlecentrifugal blower possessing a relatively high inertia impellerrequires a more complicated control system for quickly responding tofluctuations in gas flow demand.

Utilizing the multiple blower system of this invention for meeting thegas flow requirements of a fuel cell enables the system to benefit fromboth low inertia impellers for control as well as low drive motor rpmand power draw to provide required gas flow and pressure. Controllingone or more blower units in the system to provide a major portion of thetarget gas pressure and gas flow, e.g., 60-90% of the target gaspressure and gas flow, enables the remainder of the target gas pressureand gas flow to be provided by one or more other blower units in thesystem. The result of splitting the task of providing target gas flowsand pressures between at least two integrated, i.e., interconnected,centrifugal blowers in accordance with the invention results in suchflows and pressures being reached in less time and with greater accuracythan is possible with a single centrifugal blower unit. Additionally,the power draw and noise level are low in the blower system of theinvention since the blower impellers do not require high rpm for theiroperation.

Thus, in its integrated, or interconnected, arrangement of multiplecentrifugal blowers inherently possessing smaller inertial forces than asingle centrifugal blower of comparable gas flow capability, thecentrifugal blower system herein provides improved response times andcontrol over a broad range of gas pressure and gas flow requirementsthan that of a single centrifugal blower unit.

These and other novel features and advantages of this invention willbecome more apparent from the following detailed description andaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrate, respectively, a perspective view of a dual blowersystem of the invention in the 0° gas flow configuration with a sectionof the duct cutaway to show a portion of the inlet and impeller of thesecond blower unit;

FIG. 1B illustrates a plan view of the dual blower system of 1A;

FIGS. 2A and 2B illustrate, respectively, a perspective view and planview of a dual blower system of the invention in the 90° gas flowconfiguration;

FIGS. 3A and 3B illustrate, respectively, a perspective view and planview of a dual blower system of the invention in the 180° gas flowconfiguration;

FIGS. 4A and 4B illustrate, respectively, a perspective view and planview of a dual blower system of the invention in the 270° gas flowconfiguration;

FIGS. 5A, 5B, 5C and 5D are external side views of dual blower systemsof the invention with pitch angles of the outlet of the first blowerunit relative to the inlet of the second blower unit of, respectively,0°, 30°, 60° and 90°;

FIGS. 6A, 6B and 6C illustrate, respectively, perspective, plan and sideelevation views of a triple blower system of the invention in which thecombined outlet streams of first and/or second blower units areintroduced into the inlet of a third blower unit;

FIGS. 7A, 7B and 7C illustrate, respectively, perspective, plan and sideelevation views of a triple blower system of the invention in which theoutlet stream of a first blower unit is introduced into the inlet of asecond and/or third blower unit;

FIG. 8 is a perspective view of a triple blower system in accordancewith the invention in which the outlet stream of a first blower unit isintroduced into the inlet of a second blower unit and the outlet streamof the second blower unit is introduced into the inlet of a third blowerunit;

FIG. 9 is a perspective view of a dual blower system in accordance withthe invention in which the first blower unit possesses a larger impellerthan that of the second blower unit;

FIG. 10 is a perspective view of a dual blower system in accordance withthe invention in which the blower units are separated from each other;

FIG. 11A is a diagrammatic illustration of a blower control system for adual blower system in accordance with the invention;

FIG. 11B is a logic flow diagram for the dual blower control system ofFIG. 10A;

FIG. 12 is a graphic comparison of the typical performance,respectively, of a dual blower system in accordance with the inventionand a single blower system of comparable gas flow capability;

FIGS. 13A and 13B are graphic presentations of flow rate and pressuredata for dual blower systems of the invention having pitch angles,respectively, of 0°, 30° and 60°;

FIGS. 14A and 14B illustrate, respectively, perspective and plan viewsof a tubular SOFC assembly possessing separate dual blower systems ofthe invention for providing, respectively, air and fuel flow to theassembly;

FIG. 14C is a diagrammatic illustration of a cross section of anindividual tubular fuel cell in the tubular SOFC assembly of FIGS. 12Aand 12B;

FIGS. 15A and 15B illustrate, respectively, perspective and plan viewsof a planar SOFC assembly possessing separate dual blower systems of theinvention for providing, respectively, air and fuel flow to theassembly; and,

FIG. 15C is a diagrammatic illustration of a cross section of anindividual planar fuel cell in the planar SOFC assembly of FIGS. 13A and13B.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIGS. 1A and B, in one embodiment of the centrifugal blowersystem of the invention, dual centrifugal blower system 10 includes afirst centrifugal blower unit 11 connected to a second centrifugalblower unit 12 through duct 13. First blower unit 11 includes a casing14 having an axial inlet 15 and a radial outlet 16, an impeller 17disposed within casing 14 for drawing a gaseous medium at a firstpressure into axial inlet 15 and expelling gaseous medium at a secondhigher pressure through radial outlet 16 and an electric motor 18 fordriving impeller 17. Second blower unit 12 includes a casing 19 and, asshown by the cutaway section of duct 13 in FIG. 1A, an impeller 20disposed within casing 19 and driven by electrical motor 21 and an axialinlet 22 for receiving gas medium discharged from outlet 16 of firstblower unit 11. Second blower unit further includes a radial outlet 23and outlet gas stream housing 24.

The arrows in FIGS. 1A and 1B and in the other embodiments of theinvention illustrated in other figures herein indicate the generaldirection of the gas stream through the radial outlet of each blowerunit in the series of blowers constituting the blower system. As shown,e.g., in FIG. 1B, the trajectory of the gas stream expelled throughoutlet 16 of first blower unit 11 and the trajectory of the gas streamexpelled through outlet 23 of second blower unit 12 are not parallel totheir respective outlets but are at some angle thereto. By arranging thegeometry of duct 13 to receive the gas stream discharged through outlet16 in such a manner that the stream remains approximately parallel tothe interior walls of the duct, it is possible to prevent or reduce theturbulence that would otherwise occur were the stream to impinge uponthese walls. Turbulence is advantageously minimized or avoided so as toreduce or eliminate it as a source of back pressure in the blowersystem. For this same reason, it is advantageous to arrange the angle ofgas stream housing 24 so that its interior walls will be approximatelyparallel to the trajectory of the gas stream discharged through outlet23 of second blower unit 12. The optimum geometry of the interior wallsof duct 13 relative to the trajectory of its gas stream and the angle ofoffset of gas stream housing 24 can be readily determined for a givengas blower system employing routine experimentation. In the gas blowersystem shown in FIGS. 1A and 1B, interior, or guiding, surfaces of duct13 and interior, or guiding, surfaces of gas stream housing 24 can bepitched at an angle α of from 12° to 20°, and preferably from 14° to18°, relative to outlets 16 and 21.

The embodiments of the dual blower systems of FIGS. 2A, 2B, 3A, 3B, 4Aand 4B, are similar in structure to the dual blower system illustratedin FIGS. 1A and 1B except for the orientation of the outlet of secondblower unit 12 relative to the outlet of first blower unit 11. In theblower system of FIGS. 1A and 1B, the angle of orientation is about 0°.In the blower system of FIGS. 2A and 2B, this angle is about 90°, in theblower system of FIGS. 3A and 3B the angle is about 180° and in theblower system of FIGS. 4A and 4B the angle is about 270°. Allorientation angles are, of course, contemplated with the optimum angleof orientation for a given centrifugal blower system being made todepend upon the specific use to which the blower system is to be put.

Another angle of significance in the centrifugal blower system of theinvention is the angle of pitch of the outlet of the first blowerrelative to the inlet of the second blower. In the embodiments of blowersystems illustrated in FIGS. 5A-5D, the approximate angle is 0° in FIG.5A, 30° in FIG. 5B, 60° in FIG. 5C, and 90° in FIG. 5D. As in the caseof the blower unit orientation angles referred to above, these blowerpitch angles can assume values throughout the entire range of 0°-180°,again, with the optimum pitch value of a given blower system dependingon specific application requirements.

Thus far, dual centrifugal blower systems have been disclosed with theoutput of the first blower being introduced into the inlet of the secondblower and with each of the blowers having about the same range of gaspressure and gas flow output capability. The basic configuration of dualblower systems can be represented as “1 into 2” meaning that gasdischarged from the first blower is introduced into the inlet of thesecond blower. However, as those skilled in the art will readilyrecognize, numerous other arrangements are within the scope of thisinvention.

Other embodiments of the centrifugal blower system herein include thosewith three, four and even a greater number of blower units, those inwhich the discharge from two or more blowers is introduced into theinlet of a single blower and those in which the discharge of a singleblower is introduced into the inlets of two or more blowers. Blowersystems of the foregoing kind can be designated, e.g., “1 into 2 into3”, etc., where the gas discharge stream of a preceding blower unit isducted into the inlet of the following blower unit in the series, “1 and2 into 3”, etc., where the discharged streams of first and second blowerunits are commonly ducted into the inlet of a third blower unit and “1into 2 and 3” where the discharge stream of a first blower unit isducted into second and third blower units. In blower systems in which agas stream of one blower is combined with the gas stream of anotherblower or a single blower stream is divided into two separate streams,valving may be provided to regulate the various gas flows in thesesystems.

In the centrifugal blower system 60 illustrated in FIGS. 6A, 6B and 6C,the gas discharged from each of blower units 61 and 62 is introduced viaduct 63 into the inlet of blower unit 64. Centrifugal blower system 60is therefore an example of the “1 and 2 into 3” configuration referredto above. This configuration enables control to be achieved whereby thegas flow capability of a single relatively large blower is obtained withthe quick response characteristics of several smaller blowers.

FIGS. 7, 7B and 7C show centrifugal blower system 70 with the output ofsingle blower unit 70 being introduced into blower units 72 and 73 viacommon duct 74, an example of a “1 into 2 and 3” arrangement of blowerunits. This configuration of blower units enables use of a singleprimary gas pressure and gas flow supply blower with individual blowersdownstream to provide more accurate control of two separate gasdischarge streams.

In the embodiment shown in FIG. 8, the discharge stream from firstblower unit 81 of triple blower system 80 is introduced via duet 82 intosecond blower unit 83 with the discharge stream of blower unit 82 beingintroduced via duct 84 into third blower 85, such illustrating the “1into 2 into 3” configuration referred to above. This successivearrangement of three blowers permits blowers 83 and 85 to quickly andaccurately respond to target gas pressure and has flow requirements thegreater part of which are provided by blower unit 81.

Further included within the scope of this invention are thosecentrifugal blower systems in which one or more blower units differ fromone or more others in the system in their range of gas pressure and gasflow output capability. Such an embodiment of gas blower system isillustrated in FIG. 9. Dual centrifugal gas blower system 90 possesses afirst blower unit 91 of relatively large gas pressure and gas flowcapability with the gas stream expelled therefrom being introduced viaduct 92 into smaller blower unit 93. This arrangement of blowers ofdiffering size enables fine adjustment of higher gas flow rates. Wheregas flow requirements exceed that which can be achieved with a blowersystem in which the blower units are of approximately the samecapability, the larger capacity blower unit can be supplemented by thelower capacity unit. This permits a greater range of gas flow whilestill realizing the quicker and more accurate flow controlcharacteristics of the centrifugal blower system of this invention.

In all of the centrifugal blower systems of the invention, theindividual blower units, their interconnecting duct(s) aside, need notbe in direct contact with each other but can be separated by a distance.Placing one or more blowers in the blower system of the invention at aseparate location can be of advantage when optimal packagingconsiderations for a particular application favor such an arrangement.An embodiment of a blower of this type is shown in FIG. 10 where, indual centrifugal blower system 100, first blower 101 is separated fromsecond blower 102 by nearly the length of tubular duct 103.

The dimensions, voltage, power draw, impeller speed, air flow, noiselevel as well as other characteristics of a particular blower unitutilized in the centrifugal blower system of the invention can varywidely depending on gas pressure and gas flow requirements and end-useapplication. The following table lists some typical characteristics fora range of useful blower units:

Static Rating Power Power Air Pressure Size Voltage Current ConsumptionSpeed Flow (Inch- Noise Weight (mm) (VDC) (AMP) (WATTS) (RPM) (CFM)Water) (dBA) (g)  35 × 35 × 7 12 0.065 0.8 6300 0.9 0.27 22.2 8  45 × 45× 20 12 0.04 0.48 3500 4.6 0.22 21 22.64  50 × 50 × 15 12 0.17 2.2 60004.7 0.97 42.2 30  50 × 50 × 15 12 0.1 1.2 5000 4.0 0.67 39.8 30  50 × 50× 15 12 0.06 0.7 4000 3.0 0.40 33.4 30  50 × 50 × 15 12 0.044 0.5 30002.3 0.16 27 30  50 × 50 × 20 12 0.124 1.5 5200 5.7 0.66 35 33  50 × 50 ×20 12 0.104 1.3 4800 5.2 0.56 33 33  50 × 50 × 20 12 0.088 1.1 4400 4.80.46 30 33  60 × 60 × 15 12 0.105 1.3 4800 5.2 0.44 40.5 45  60 × 60 ×15 12 0.07 0.8 4200 4.7 0.32 36 45  60 × 60 × 15 12 0.04 0.5 3200 3.50.18 29 45  60 × 60 × 25 12 0.14 1.7 3600 7.3 0.58 32.4 55  75 × 75 × 3012 0.3 3.6 3400 13.6 0.6 43.5 86.5  75 × 75 × 30 12 0.23 2.8 3000 12.30.48 40.5 86.5  75 × 75 × 30 12 0.13 1.5 2400 9.6 0.27 33.9 86.5  75 ×75 × 30 12 0.08 1.0 1900 7.5 0.15 28 86.5  75 × 75 × 30 24 0.17 4.1 340013.6 0.6 43.5 86.5  75 × 75 × 30 24 0.14 3.4 3000 12.3 0.48 40.5 86.5 75 × 75 × 30 24 0.08 1.9 2400 9.6 0.27 33.9 86.5  75 × 75 × 30 24 0.051.2 1900 7.5 0.15 28 86.5  97 × 97 × 33 12 560 6.7 3600 26.7 0.76 54.3183  97 × 97 × 33 12 0.72 9.6 3600 30.5 0.92 55.8 185  97 × 97 × 33 120.56 6.7 3200 26.6 0.65 53.1 185  97 × 97 × 33 12 0.30 3.6 2700 22.40.43 50 185  97 × 97 × 33 24 0.39 9.4 3600 30.5 0.92 55.8 185  97 × 97 ×33 24 0.25 6.0 3200 26.6 0.65 53.1 185  97 × 97 × 33 24 0.16 3.8 270022.4 0.43 50 185  97 × 97 × 33 48 0.17 8.2 3600 30.5 0.92 55.8 185  97 ×97 × 33 48 0.13 6.2 3200 26.6 0.65 53.1 185  97 × 97 × 33 48 0.09 4.32700 22.4 0.43 50 185 120 × 120 × 32 12 755 9.06 2800 38.7 1.14 55.8 242120 × 120 × 32 12 0.82 9.8 2500 35.9 0.89 53.8 250 120 × 120 × 32 120.45 5.4 2100 31.4 0.64 49.6 250 120 × 120 × 32 24 0.38 9.1 2500 35.90.89 53.8 250 120 × 120 × 32 24 0.24 5.8 2100 31.4 0.64 49.6 250 120 ×120 × 32 24 0.38 9.1 2500 35.9 0.89 53.8 250 120 × 120 × 32 24 0.24 5.82100 31.4 0.64 49.6 250 120 × 120 × 32 48 0.12 5.8 2100 31.4 0.64 49.6250

It will, of course, be recognized that the invention is not limited toblower units possessing the forgoing characteristics but can utilize anycentrifugal blower unit having lesser or greater dimensions, voltage andpower requirements, impeller rpm, gas pressure and gas flowcapabilities, etc., than those listed in the table.

FIGS. 11A and 11B illustrate, respectively, a blower control system of acentrifugal blower system of the invention and a diagrammaticrepresentation of its control logic. As those skilled in the art willrecognize, these blower control operations can be carried out by asuitably programmed microprocessor.

FIG. 12 compares the typical flow rate performance of independentlycontrolled first and second blowers in a dual centrifugal blower systemsuch as that shown in FIGS. 1A and 1B with a conventional larger singlecentrifugal blower of approximately equivalent gas flow capability. Asthe data plots show, the overdamping of the single blower which isrequired to avoid or suppress overshooting target gas flows resulted ina longer period of time to reach both low target flow and high targetflow in contrast to the considerably faster times for achieving thesetarget flow levels employing the multiple interconnected centrifugalblower system of the invention.

FIGS. 13A and 13B are graphical presentations of, respectively, gas flowrate and gas pressure performance characteristics for dual blower systemconfigurations of the invention in which the pitch angles of the blowerunits are 0°, 30° and 60° (as shown in FIGS. 5A, 5B and 5C).

The centrifugal blower system of this invention can manage gas flowrequirements for a variety of applications. FIGS. 14A, 14B, 15A and 15Billustrate the use of the blower system of the invention to provide andmediate gas flows in an SOFC assembly of the tubular type (FIGS. 14A and14B) and planar type (FIGS. 15A and 15B).

In tubular SOFC assembly, or stack, 140 of FIGS. 14A and 14B, firstblower system 141 provides a gaseous fuel, e.g., hydrogen, to manifold142 for distribution to the interior array 143 of tubular SOFC elements.Each tube in array 143 can be of known or conventional construction and,as shown in FIG. 14C, possesses an innermost fuel-contacting anodelayer, intermediate electrolyte layer and outer cathode layer. Secondblower system 144 distributes air, initially at ambient temperature, tomanifold 145 from which it is released to provide a source of oxygen forthe cathode component of each tubular SOFC element. The air enteringmanifold 145 gains heat from the hot combustion gases exiting tailburner 146 into heat exchanger 147. The dotted lines show the flow pathof the heated air existing the outlets of manifold 145, passing throughthe SOFC array 143 and into tail burner 146 where it provides oxygen tosupport combustion of unspent fuel present in the exhaust gas emergingfrom the tubular SOFC elements into exhaust manifold 148 and from thereinto the tail burner. Finally, the hot combustion gases enter heatexchanger 147 where they serve to preheat incoming air provided by firstblower system 141 as previously indicated.

The construction and operation of the planar SOFC assembly shown inFIGS. 15A and 15B is much the same as that described above for thetubular SOFC assembly of FIGS. 14A and 14B the principal differencebeing the use of planar SOFC elements. As shown in FIG. 15C, each planarSOFC element in array 151 includes anode, electrolyte, cathode andinterconnect components.

Although the invention has been described in detail for the purpose ofillustration, it is understood that such detail is solely for thatpurpose, and variations can be made therein by those skilled in the artwithout departing from the spirit and scope of the invention which isdefined in the claims.

What is claimed is:
 1. A centrifugal blower system comprising: a) aseries of blower units, each blower unit in the series of blower unitscomprising a casing having an axial inlet and a radial outlet, asubstantially tangential wall of the casing extending to the outlet, animpeller disposed within the casing for drawing a gaseous medium at afirst pressure into the axial inlet and expelling gaseous medium at asecond higher pressure through the radial outlet along a trajectoryinclined to the substantially tangential wall, and a motor for drivingthe impeller; b) a duct directly connecting the radial outlet of atleast one upstream blower unit in the series of blower units with theaxial inlet of at least one successive downstream blower unit in theseries of blower units; and, c) at least one gaseous medium-directingstructure selected from the group consisting of the duct having interiorwalls configured to be substantially parallel to the trajectory of thegaseous medium expelled from the radial outlet of a blower unit in theseries of blower units to which the duct is connected, and a gas streamhousing for receiving the gas stream from the radial outlet of asuccessive blower unit in the series of blower units, interior walls ofthe gas stream housing being configured to be substantially parallel tothe trajectory of the gaseous medium expelled from the radial outlet. 2.The centrifugal blower system of claim 1 in which the orientation of theradial outlet of one blower unit in the series of blower units to theaxial inlet of a successive blower unit in the series of blower units isapproximately 0°, approximately 90°, approximately 180° or approximately270°.
 3. The centrifugal blower system of claim 1 in which the angle ofpitch of the radial outlet of one blower unit in the series of blowerunits to the axial inlet of a successive blower in the series of blowerunits is approximately 0°, 30°, 60° or 90°.
 4. The centrifugal blowersystem of claim 2 in which the angle of pitch of the radial outlet ofone blower unit in the series of blower units to the axial inlet of asuccessive blower in the series of blower units is approximately 0°,30°, 60° or 90°.
 5. The centrifugal blower system of claim 1 wherein atleast one blower unit in the series of blower units has greater gaspressure and gas flow capability than another blower unit in the seriesof blower units in the centrifugal blower system.
 6. The centrifugalblower system of claim 1 wherein the interior walls of the gas streamhousing are at an angle α of from 12° to 20° relative to a radial outletof a blower unit to which the gas stream housing is connected.
 7. Thecentrifugal blower system of claim 1 wherein the angle of pitch of theradial outlet of one blower unit in the series of blower units to theaxial inlet of a successive blower unit in the series of blower units isapproximately 0° and the interior walls of the gas stream housing are atan angle α of from 12° to 20° relative to a radial outlet of a blowerunit to which the gas stream housing is connected.
 8. The centrifugalblower system of claim 1 which includes a microprocessor configured toindependently control the operation of blower units in the series ofblower units such that operation of at least one blower unit in theseries of blower units provides from 50% to 90% of a target gaseous flowof the centrifugal blower system and the operation of at least one otherblower unit in the series of blower units provides the balance of thetarget gaseous flow of the centrifugal blower system.
 9. The centrifugalblower system of claim 8 wherein the interior walls of the gas streamhousing are at an angle α of from 12° to 20° relative to a radial outletof a blower unit to which the gas stream housing is connected.
 10. Thecentrifugal blower system of claim 8 wherein the angle of pitch of theradial outlet of one blower unit in the series of blower units to theaxial inlet of a successive blower unit in the series of blower units isapproximately 0° and the interior walls of the gas stream housing are atan angle α of from 12° to 20° relative to a radial outlet of a blowerunit to which the gas stream housing is connected.
 11. A method ofcontrolling the operation of the blower system of claim 1 whichcomprises: a) operating at least one blower unit in the series of blowerunits to provide from 50% to 90% of a target gaseous flow of thecentrifugal blower system; and, b) operating at least one other blowerunit in the series of blower units to provide the balance of the targetgaseous flow of the centrifugal blower system.
 12. The centrifugalblower system of claim 1 further comprising a microprocessor configuredto independently control the operation of the blower units in the seriesof blower units.